1. Field of the Invention
This invention relates generally to speed limiting devices for wind turbines, and relates more particularly to a hydraulic speed control device for limiting the speed of a wind turbine rotor by controlling the deployment of braking tabs.
2. Description of the Relevant Art
Wind turbines have been used as a source of power for many years. One problem associated with the operation of wind turbines is that of speed control. Controlling the speed of a wind turbine rotor is necessary in order to avoid the damaging effects of high velocity winds on the wind turbine and its supporting structure. Most wind turbine rotors rotate at a speed that is directly related to the speed of the driving winds; that is, the faster the winds blow, the faster the rotor rotates. Structural damage due to over-stressing the turbine blades and rotor can occur if the rotational speed of the wind turbine is allowed to increase without limit.
Various devices and adjusting mechanisms have been used in the prior art to control the speed of horizontal axis wind turbines rotors. One approach to controlling the speed of rotors is to change the angle of attack of the blades so that the incident wind strikes the blades at an unfavorable angle, thus establishing an inefficient conversion of wind energy into rotational energy of the rotor. Instead of rotating each entire blade, another approach is to rotate a braking tab positioned at the tip of each blade in order to brake the rotor. Some implementations of this latter approach have utilized the centrifugal force generated by the rotor rotation as a force that tends to move the tabs to their braking positions, while opposing that movement with a mechanism that releases the tabs for movement to their braking positions only when the maximum desired rotor speed has been exceeded.
One example of a speed control device using blade tabs is disclosed in U.S. Pat. No. 4,374,631 issued Feb. 22, 1983 to Barnes, and entitled "Windmill Speed Limiting System Utilizing Hysteresis." In the Barnes speed control device, rotatable braking tabs are coupled to the tips of rotor blades through cam mechanisms that impart a twist to the tabs as they move radially outward due to centrifugal force. The outward movement of the tabs is opposed by a spring mechanism mounted in the hub of the rotor. When the rotational speed of the rotor exceeds a certain level, the spring mechanism releases the tabs, which then rotate to their braking position to slow the rotor.
While these and other prior art speed control devices are somewhat effective in controlling the speed of wind turbine rotors, some disadvantages are associated with their use. One disadvantage is that the rotational speed at which the blades or tabs are reset or realigned to their operational positions is often not independently adjustable from the rotational speed at which the blades or tabs are deployed to their braking positions. Another disadvantage of some prior art speed control devices is that the reset speed is just below the deployment speed, so that in high winds the tabs oscillate between the operational and braking positions without effectively slowing the wind turbine rotor to a safe speed. Another disadvantage is that all of the tabs may not be deployed in unison, which can create a dynamic imbalance of the rotor that causes structural damage. Still another disadvantage is that the speed control mechanism may fail in such a way that one or more of the tabs are locked into their operational positions and can not deploy to slow the rotor.